Abiotic stressors significantly limit crop production worldwide. Cumulatively, these factors are estimated to be responsible for an average 70% reduction in agricultural production (Bresson, 1999).
Drought stress, in particular, not only causes a reduction in the average yield for crops but also causes yield instability through high interannual yield variation. Globally, about 35-40% of arable land falls under arid or semiarid classification. Even in non-arid regions where soils are nutrient-rich, drought stress occurs regularly for brief periods or at moderate levels. Moreover, it has been predicted that in the coming years rainfall patterns will shift and become more variable due to increased global temperatures.
U.S. studies have shown that the ten most important kinds of cultivated plants (corn, soybeans, wheat, tomatoes, etc.) produced only about 50% of the genetically possible yields on average per year; two thirds of the losses were due to the frequent combination of heat stress and water shortage (G. Schütte, S. Stirn, and V. Beusmann, Transgene Pflanzen-Sicherheitsforschung, Risikoabschätzung and Nachzulassungs-Monitoring. Birkhäuser Verlag AG, Basel-Boston-Berlin, 2001).
Plants are sessile and have to adjust to the prevailing environmental conditions of their surroundings. This has led to their development of a great plasticity in gene regulation, morphogenesis, and metabolism. Adaptation and defense strategies involve the activation of genes encoding proteins important in the acclimation or defense towards the different stressors. Some of the molecular responses to abiotic stress factors such as drought are specific, but it has also been shown that similar genes are activated by several stressors (Royal Society of London, Transgenic Plants and World Agriculture, 2000, National Academy Press, Washington, D.C.). It is believed that about 15 percent of a plant's genome is devoted to stress perception and adaptation (see e.g., Cushman and Bohnert, 2000).
Earlier work on molecular aspects of abiotic stress responses was accomplished by differential and/or subtractive analysis (e.g., see Bray, 1993, Shinozaki and Yamaguchi-Shinozaki, 1997, Zhu et al., 1997, Thomashow, 1999). Other methods include selection of candidate genes (e.g., selection of genes from a particular known module and analyzing expression of such a gene or its active product under stresses, or by functional complementation in a stressor system that is well defined, see Xiong and Zhu, 2001). Additionally, forward and reverse genetic studies involving the identification and isolation of mutations in regulatory genes have also been used to provide evidence for observed changes in gene expression under stress or exposure (Xiong and Zhu, 2001).
Activation tagging can be utilized to identify genes with the ability to affect a trait. This approach has been used in the model plant species Arabidopsis thaliana (Weigel et al., Plant Physiol. 122:1003-1013 (2000)). Insertions of transcriptional enhancer elements can dominantly activate and/or elevate the expression of nearby endogenous genes. This method can be used to select genes involved in agronomically important phenotypes, including stress tolerance.